Late Blight Prevention for Portland Tomatoes in Late May

Rhody trusses snap off, dahlias get pinched, tomato foliage stays bone dry. Portland's dry season starts Monday.

Late blight prevention for Portland tomatoes moves from theory to daily practice this week, because nighttime lows are finally holding above 50°F and the last May rains are on their way out. That combination, warm wet nights followed by muggy mornings, is exactly what the Phytophthora infestans spores wait for. Set up your beds correctly in the next seven days and you buy yourself a much easier July.

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Frequently Asked Questions

When does late blight usually show up on Portland tomatoes?

Late blight typically appears in Portland gardens between late June and mid July, once nighttime temperatures hold above 50°F and morning humidity stays above 90 percent. The infection window opens earlier in wet years and in shaded gardens with poor airflow. Prevention set up in May, drip irrigation and heavy mulching, is far more effective than any spray applied after symptoms show.

Can I still plant tomatoes in Portland during the last week of May?

Yes, late May is actually prime planting time in Zone 8b once soil temperature reaches 60°F at 4 inches deep. Choose 4 to 6 inch transplants, bury them up to the first true leaves to build root mass, and water in with a diluted fish emulsion. Varieties like Legend, Stupice, and Oregon Spring are bred for Portland and outrun blight better than most heirlooms.

Should I prune my rhododendron now that it's finished blooming?

Only very lightly. Snap off the spent flower trusses at their base to redirect energy into next year's buds, and cut back any obviously damaged or crossing branches. Heavy structural pruning on rhododendrons should wait until late June at the latest, because flower buds for 2027 begin setting in July and any cuts after that remove next year's show.